'float' 和 'NoneType' 的實例之間不支持 '>'

Question

我運行以下代碼來定義一些基本對象：

import logging

class State:
    """A class representing a US state."""
        
    def __init__(self, name, postalCode, population):
        self.name = name
        self.postalCode = postalCode
        self.population = population
        
    def __str__(self):
        """Readable version of the State object"""
        return 'Name: ' + self.name + ", Population (in MM): " + str(self.population) + ", PostalCode: " + self.postalCode
    
    def increase_population(self, numPeople):
        """Increases the population of the state."""
        self.population += numPeople
        print("Population of",self.name,"increased to",self.population,"million.")
        
    def decrease_population(self, numPeople):
        """Decreases the population of the state."""
        try:
            if (numPeople > self.population):
                raise ValueError("decrease_population(self, numPeople): Invalid value for population reduction")
            else:
                # decrease the population by the value in variable numPeople
                self.population -= numPeople

                # Print new population
                print("Population of",self.name,"decreased to",self.population,"million.")
        except Exception as e:
            logging.exception(e)            

# Test Cases             
# Create an instance of State 'il' corresponding to Illinois, with a postal code of IL and a population of 12.8 million
il = State("Illinois","IL",12.8)

# use the given method to increase the population of il by 1 million
il.population = State.increase_population(il, 1)

# use the given method to decrease the population of il by 1.5 million
il.population = State.decrease_population(il, 1.5)

但是我收到一個 TypeError: '>' not supported between 'float' and 'NoneType'錯誤在線

            if (numPeople > self.population):

我試圖在 SO 上找到解決方案，但沒有找到有效的答案。

Answer 1

看起來您正在混合實例方法，即自我和靜態/類方法。

將您的代碼更改為

il = State("Illinois", "IL", 12.8)

# use the given method to increase the population of il by 1 million
il.increase_population(1)

# use the given method to decrease the population of il by 1.5 million
il.decrease_population(1.5)

實例方法increase_population返回 None 這使得

il.population = State.increase_population(il, 1 )

到 None 另外increase_population(il, 1)是實例方法，當您將實例名稱傳遞給它並像 static_method 一樣調用時，可以像il.increase_population(1)一樣調用它

與方法decrease_population相同的解釋